That's interesting - it looks to be a variation on the Vigenère square and the tabula recta... though instead of one key that is repeated (HIGHWAYHIGHWAY...) it

That's interesting - it looks to be a variation on the Vigenère square and the tabula recta... though instead of one key that is repeated (HIGHWAYHIGHWAY...) it also shuffles the alphabet used with another key... and has the possibility of shifting the position of that within the shuffled alphabet... though I'm not sure if that gains anything significant (instead of HIGHWAYHIGHWAY it becomes JLHJXUZJLHJXUZ which is a shift of one column and most letters are a shift of one except A -> U because that's the key shuffled alphabet). https://sites.google.com/site/cryptocrackprogram/user-guide/cipher-types/substitution/quagmire?pli=1 https://sites.google.com/site/cryptocrackprogram/user-guide/... https://crypto.stackexchange.com/questions/35232/how-to-break-a-quagmire-3-cipher https://crypto.stackexchange.com/questions/35232/how-to-brea... https://eprint.iacr.org/2022/1475.pdf https://eprint.iacr.org/2022/1475.pdf It's gonna be added to my "things to play with when nothing else is pending."